Reception of signals



April 6, was. 1,57%,253

I E. SINGER RECEPTION 0F SIGNALS Filed May 21 1923 Patented Apr. 6, 1926.

UNITED STATES PATENT OFFICE EMANUEL SINGER, OF NEW YORK, N. Y., ASSIGNOR T0 WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

RECEPTION OF SIGNALS.

Application filed May 21, 1923. Serial No. 640,289.

To all whom it may concern:

Be it known that I, EMANUEL SINGER, a citizen of the United States, residing at New York, in thecounty of New York, State of New York, have invented new and useful Improvements in the Reception of Signals, of which the following is a full, clear, concise, and exact description.

The present invention relates to signaling by means of electrical waves, and more particularly to the reception of the signals so transmitted. Still more particularly the invention relates to amplifying and detecting the signals, and in such a way that a high degree of amplification is obtained with a comparatively small amount of apparatus.

It is known that a certain factor of am plification applied to a radio or carrier frequency Wave before detection gives a louder signal than'if the same factor of amplification is applied to the signal wave after it is detected. If the detector obeys the square 7 law relation such that output current variations are proportional to the square of the input voltage variations, then an amplification of three fold, for example, in the radio or carrier frequency wave is equivalent to a nine-fold amplification of the signal after detection.

This advantage can be realized in practice in radio reception, particularly where the received radio waves are relatively weak so that the impressed. voltages are relatively small, and where in addition, the circuit as a whole is efiicient at radio frequencies. The latter requirement has been found to give considerable difiiculties at short wave lengths 'on account of the losses in the tubes and other parts of the circuit due to capacity and other effects. It has accordingly been proposed to step the frequency of the received radio waves down to a frequency level at which they can be readily amplified and to carry out the amplification at this lower frequency level. The radio frequency wave with its signal variations is in effect reduced to some intermediate frequency wave with the signal variations upon it, and this intermediate frequency wave after amplification is detected for the signal. The troublesome capacity effects are less marked at-the intermediate frequency than at the radio frequency. and the amplification can be more P efficiently carried out. If the detector serv- 'ing to derive the signals from the intermediate frequency wave obeys the square law relation mentioned above the increase in am plitude to the selected signal component due to each intermediate frequency amplification is proportional to the square of such amplification rather than merely the amplification ratio itself, and the same advantage, therefore, results from the intermediate frequency a-mplification as from radio frequency amplification.

An object of the invention is to obtain a high degree of amplification of the incoming wave by a relatively small amount of amplifier apparatus before the signal is detected. v p

A related object is to amplify the Waves a plurality of times by thevsame amplifier before detection of the signal in amanner to avoid the production of sustained oscillations.

Other objects and the various featuresprovided by the invention for carrying out the objects will be clear as the description proceeds. I

Briefly and specifically stated, the invention utilizes the same amplifier for amplification at each of a plurality of frequencies higher than the signal frequency, such as at the frequency of the radio or carrier wave as received and at one or more intermediate frequencies. After the wave to be amplified passes through the amplifier circuit it is subected to a frequency-reducing process and the wave of reduced frequency with signal variations is passed through the amplifier circuit again, whereupon its frequency may be again stepped down, and so on. By this method, the wave may be given a large amplification at each frequency level without the danger of producing self-sustained 0scillations that would be incurred if the same frequency of wave were passed successively through. the amplifier circuit. This results in very high signal amplification, since the successive amplifications are carried out be- 9 fore the final signal detection, and the advantages therefore, of this amplification over amplification of the detectedsignal. as above pointed out, are present to the fullest extent that may be desired. Furthermore, as the 9 amplitude of the wave is increased its frequency is lowered so that the tendency'of" the circuit to develop free oscillations 1s always kept low without limitin the degree of amplification as in the case w re'the amno plification is all carried out at the frequency of the received radio wave. A feature of the invention is a simple type of circuit used for connecting the amplifier tubes so as to enable them to operate at a radio frequency, an intermediate frequency and an audio frequency without troublesome interference and excessive loss of the high frequencies in the amplifier circuit.

The invention enables the advantages of both radio frequency and intermediate frequency amplification to be realized, and combines the relatively low amplification that is possible at the very high radio frequencies with the relatively larger amplification that is possible at the intermediate frequencies in the order resulting in greatest efiiciency. I

The various objects and features of the invention will be clear from the more detailed description to follow: In the description reference will be made to the accompanyin drawing illustrative of one form which are invention may take. 7

The drawing shows a receiving circuit of a radio system in which the waves intercepted by the receiving antenna, 1, are re eeived, amplified and detected in accordance with the invention and the signal is made audible in the receivers 2. It will be understood that the invention is equally applicable to other types of signaling systems than radio, and the antenna 1, for example, may be replaced by a line.

In the form shown in the drawing, the tubes 3, 4, 5 and 6 are amplifying electron discharge tubes of well known type and serve to amplify the received waves in the manner to be described. It will be obvious to employ a less or a greater number of amplifying devices than the four that are shown and in some cases one amplifying tube may be sufficient. Associated with the amplifier, two detectors 7 and 8 are indicated, these being of the well known threeelement detector tube type. lVhile it is not deemed necessary in all cases to employ two separate detectors to perform the functions hereinafter described in connection with these detectors, the two detectors have been shown for purposes of illustration, together with selective circuits for separating the waves upon which these detectors operate, but the invention is not to be construed as limited to the arrangement as shown. 7

A local oscillator 9 is also provided, this being indicated as of the type disclosed in Hartley Patent No. 1,356,763, October 26, 1920.

The construction of the circuit, as a whole, and the cooperation of the various parts will be made clear in the following description of the operation of the circuit in the tenna 1 is impressed upon the resonant circuit 10 which is preferably tuned to the frequency of the received wave, and the potential across the tuning condenser is applied to the grid of the first radio frequency amplifying tube 3 in accordance with the usual practice. l/Vhen waves from distant stations are being received, the voltage thus impressed on the amplifier 3 is exceedingly minute, and in systems in which tubes are used exclusively for amplifying received radio frequencies, the actual load carried by such tubes is only a very small fraction of the load which they are capable of carrying without unduly distorting the amplified waves. The radio frequency wave impressed on the amplifier 3 receives some amplification in this tube and is impressed on the grid circuit of the tube 4 where it is further amplified and fed into the tube 5, and so on. It is assumed that the load on the tube (1 is considerably less than the maximum load which this tube is capable of carrying.

The radio frequency wave thus amplified is impressed on the grid circuit of the detector 7 which is also supplied with continuous waves from the local oscillator 9. The frequency of the locally supplied waves is adjusted to a value materially different from the frequency of the incoming waves, so that the beat frequency or difference frequency wave resulting from the well known action of the detector 7 is relatively very much higher than the signal frequency itself and is preferably inaudible. Due to the fact that the beat frequency lies between the carrier frequency and the signal frequency, it is commonly called the intermediate frequency. A frequency which has been employed as the difference frequency or intermediate frequency and has proved convenient is of the order of 50,000 cycles per second. The amplitude of the locally impressed wave may be regulated in a suitable manner by varying the constants of the oscillator and by varying the coupling between the oscillator and detector.

The detected intermediate frequency wave is selectively transmitted through the circuits 11 and 12 which are tuned to the intermediate frequency, and is impressed on the grid circuit of the amplifier 3, the potential across the tuning condenser 13 being applied between the filament and grid of this amplifier. The intermediate frequency \\';l\'\: is therefore amplified by the tubes 3, i. 5 and 6 in the manner described in connection with the received radio frequency wave. The amplitude of the intermediate frequency wave impressed on the amplifier 3 is much greater than that of the received frequency wave since the amplitude of the difference frequency wave is dependent upon the amplitude of the wave impressed on form shown. The wave received in the anthe detector from the amplifier 6, which amplitude is much greater than that of the received radio wave, and is also dependent in part on the amplitudejof the locally suplied oscillations which may be adjusted to ave any desired amplitude.

However, the tendency of the amplifier circuits to produce free oscillations and also the other difiiculties attendant upon amplification of a high frequency wave due to capacity effects and the like, arev present to a much less extent in the case of the difference frequency wave than in the case of the received radio frequency wave, so that a much larger amplitude of intermediate frequency wave can-be impressed on the amplifier without causing the circuit to sing than wouldbe possible at the radio frequency.

' The amplified intermediate frequency frequency. The small condenser 16' oifers a relatively high attenuation to waves of the intermediate frequency so that the inductance 14 together with the resonant circuit 15 on the one hand and'the small con denser 16 on the other hand provide a filtering combination which acts to selectively transmit the radio frequency wave to the detector 7 and the intermediate frequency wave j into the circuit 15, from which it is impressed on the detector 8.

The detector 8 serves to derive the signal frequency component from the amplified intermediate frequency wave and to set up across the relatively large condenser 17, potentials of the signaling frequency which are applied beween the filament and grid of the amplifier 3. The signal waves, such as speech for example, are, therefore, am-

' plified by the tubes 3, 4 and 5 and after I such amplification pass into the receivers 2 where they produce audible signals of large amplitude. The by-pass condenser 18 may be made relatively smaller than the condensers 19 and 20 so that the amplified speech currents produce small or negligible voltage'on the grid of the amplifier 6.

- It will be, noted that space current .is supplied for the amplifiers 3, 4 and 5 through the branches 21, 22 and 23 respectively. These branches cooperate with. the by-pass condensers 19, 20 and 18 respectively, to cause the variation components in the .chokevcoils. in series 1-nsa1d space -current output current ofeach amplifier to pass to the grid of the succeeding amplifier and in p v nt substautml y y f these vanasignal waves.

time

tion components from'passing through the waves of theradio frequency and also to waves of the'intermediate frequency and the This is preferably accomplished by providmg three impedances in each of the space current supply branches,

.such as the impedances 24, 25 and 26 in branch 21. .The impedance 24 immediately associated with the anode of the amplifier is a radio frequency choke coil having negllglble shunt capacity and comprising a few turns to give the necessary inductive reactance at these frequencies. The intermediate frequency waves would easily trav- Y erse a coil constructed as coil 24, and the proper impedance to waves of this fre *quency' is afforded by coil 25 which comprises a larger number of turns than coil 24 and may, to advantage, have an ironcore. The signal currents would easily traverse coil 25, however, and the necessary impedance to currents of this frequency is afforded by coil 26 which has a relatively large number of turns and may, to advantage, contain an L ironcore.

While the type of interstage connection employing choke coils and by-pass con-.-

densers has been indicated, it is within the invention to use transformer couplings betweenstages.

While provision hasbeen shown for producing'and amplifying only one intermediate frequency wave, it is obvious that'by a mere extension of the principle em loyed in the circuit as described, a plura 1ty of waves of different intermediate frequencies may. be produced and amplified by the common amplifiers, 3, 4,'ete. and it is within the invention to extend the circuit in this 7 manner.

Wat is claimed isi f I 1-. n a receiving system, a circuit for receiving a high frequency wave modified by a signal, a space discharge device having a grid circuit and a space-current supply circuit, for amplifying said wave, means to reduce the frequency of the amplified waveto an intermediate frequency, means ,to' apply said wave of intermediate frequency to the grid circuit of said device for further amplification, means to reduce the resulting amplified wave to the signal frequency level, means to a ply the wave of the signal freqrliency to t ev grid circuit of said device for rther amplification, and a plurality of Suppl; circuit, said coils being so proporto waves of said high frequency, of said intermediate frequency and of said signal frequency, respectively.

2. In an amplifying system, means to receive high frequency signal-modified waves, a vacuum tube am lifying circuit comprising, a plurality of stages, for amplifying said waves, a space-current supply branch in each of said stages, means to step down the frequency of the amplified waves to an intermediate frequency, means to apply the waves of the intermediate frequency to said amplifying circuit for further amplification, means to step down the frequency of the resulting amplified waves to the signal frequency level, means to apply the waves of signal frequency to said amplifying circuit for further amplification, and means to substantially prevent any of the amplified wave components from passing through the space current supply paths of said amplifying circuit, said last-mentioned means comprising a plurality of choke coils in series in the space current supply path in each stage, one of said coils in each path offering a high impedance to waves of said high frequency, another of said coils in each path offering a high impedance to waves of said intermediate frequency, and a third of said coils in each path offering a high impedance to waves of said signal frequency.

3. In a wave receiving system, means to receive high frequency signal modified waves, a multi-stage amplifier for amplify-v ing said waves, each stage of said amplifier comprising a space discharge tube having a cathode, anode and grid or control element, a space current supply path for each tube of said amplifier, means to reduce the frequency of the amplified waves to an intermediate frequency, means to apply waves of said intermediate frequency to said amplifier for further amplification, means to reduce the frequency of the resulting amplified waves to the signal frequency level, means to apply the waves of the signal freuency to said amplifier for further amplification, and coupling means between successive stages of said amplifier to elliciently transmit between said stages waves of said high frequency, said intermediate frequency and said signal frequency, said coupling means comprising a plurality of choke coils in series in the space current supply path in each of said stages, the one of said choke coils nearest the anode of the tube in each stage having a high impedance to waves of said high frequency, the one of said choke coils farthest from the anode of the tube in each stage having a high impedance to waves of said signal frequency and an intermediate one of said coils in each stage having a high impedance to waves of said intermediate frequency.

In witness whereof, I hereunto subscribe my name this 16th day of May, A. D., 1923.

EMANUEL SINGER. 

